How about, nobody knows? Some long-gone precursor civilisation created them for reasons unknown. And now they're around as very powerful artifacts for those they chose to bond with. This might be a bit of a cop out and tired trope, but I thought of it since you mentioned Fantasy as an inspiration for the idea.

This of course depends on whether non-human civilisations make sense in your setting.

How about, nobody knows? Some long-gone precursor civilisation created them for reasons unknown. And now they're around as very powerful artifacts for those they chose to bond with. Although this might be a bit of a cop out and tired trope.

I thought of it since you mentioned Fantasy as an inspiration for the idea (ancient artifacts and so on.)

This of course depends on whether non-human civilisations make sense in your setting.